Popup of Firefox remains on screen during some miliseconds when another window is already focused under Wayland
start wayland session
open nautilus and Firefox
start some download, click the button to show download progress
while the popup showing download progress is visible, press super key, select nautilus
Result: popup showing download progress remains visible during some miliseconds when nautilus is already focused.
It's more noticeable when my session becomes slower after some hours using my laptop.
Also reproducible with the popup of Firefox that offers to remember login data.
Watch my screencast recorded on Arch Linux running Gnome 3.28.2.